1. Field of the Invention
The present invention relates generally to front-end processing for making liquid crystal display devices. More particularly, the present invention relates to a method for making a liquid crystal display devices capable of alleviating or eliminating Mura and white pad defects.
2. Description of the Prior Art
The uses of a liquid crystal flat panel displays (LCFPD) continue to grow rapidly. Consumer items such as portable video recorders, pocket televisions, notebook computers, high-definition televisions (HDTV), and the like incorporate such displays. Based upon the continued demand for such displays, the industry has made massive capital investments to create state-of-art manufacturing lines.
Major classes of LCFPD defects encountered at the final inspection are often pixel defects or wide-area pixel defects (also known as Mura defects). Problems in the manufacturing process of the LCFPD often cause Mura defects. Because certain manufacturing problems cause certain types of Mura defects, thus identification and elimination of the manufacturing problems often leads to the reduction of Mura defects during subsequent processing runs.
Mura defects are defined as areas of illumination (pixels on the substrate) which are different, or anomalous, from the neighborhood surrounding the defect, also termed Patterned Brightness Non-Uniformity (BNU). The BNU is generally measured by an image acquisition device position normal to the LCFPD and the BNU is generally of very low contrast. Regions of a substrate often include pixels that appear either brighter or darker than pixels surrounding the region, and are classified as Mura defects when specific contrast threshold limits, BNUs, are reached or exceeded. It is important to note that the boundaries for Mura defects are not always well defined and further, within a Mura defect, the BNU may not be homogenous.
Typically, in order to manufacture a liquid crystal display panel, a metal-array forming process is first carried out, which comprises providing a clean glass substrate surface; depositing a dual-layer metal film onto the surface of the substrate; and defining and etching an array pattern (gate pattern and word line pattern) of the dual-layer metal film using conventional lithographic and metal etching methods. The dual-layer metal film may include Mo/AlNd, MoW/AlNd, or MoW/Al. The etching of the dual-layer metal film leads to slightly oblique sidewalls of the defined word lines or gate lines on the substrate. Etching gas mixtures such as oxygen/fluorine containing gases and oxygen/chlorine containing gases are used to etch Mo or MoW (the upper layer of the dual-layer metal film). Etching is often ceased by time mode (etching rate controlled). An over etch is usually carried out.
One obstacle to making a non-Mura panel is insufficient etching selectivity between the upper layer (Mo or MoW) and the lower layer (AlNd or Al) during the etching. At an array final inspect stage, circular or irregular Mura defects are often observed on the panel. According to results based on optical and electron microscopic analyses, the circular Mura defects are most likely related to rough oblique sidewalls of the etched dual-layer metal lines in a center area of the panel, while the irregular Mura defects are most likely related to small spacing between the upper layer metal and the lower layer metal.